Thermoplastic resin microporous membranes are used widely as a membrane for separation, selective permeation, isolation of substances, and the like. For example, the usage includes battery separators for lithium ion rechargeable batteries, nickel-metal hydride batteries, nickel-cadmium batteries, or polymer batteries, separators for electric double layer capacitors, various filters such as reverse osmosis filtration membrane, ultrafiltration membrane, microfiltration membrane and the like, waterproof moisture-vapor permeable clothes, and medical materials.
In particular, a polyethylene microporous membrane which exhibits ion permeability due to electrolytic solution impregnation, excellent electrical insulating properties, and a pore blocking effect, which blocks an electrical current to prevent an excessive temperature increase at a temperature of approximately 120 to 150° C. at the time of an abnormal temperature increase in a battery, is suitably used as a lithium ion rechargeable battery separator. However, if the temperature in a battery continues to increase even after the pore blocking for some reasons, the polyethylene microporous membrane may be punctured due to the shrinkage of the membrane. This phenomenon is not limited to polyethylene microporous membranes. Even in the case of a microporous membrane including another thermoplastic resin, this phenomenon cannot be avoided at a temperature equal to or higher the melting point of resin.
In particular, separators for lithium-ion batteries greatly affect battery characteristics, battery productivity and battery safety, and require good heat resistance, adhesion to electrode, permeability, membrane melt-puncture characteristics (melt-down characteristics) and the like. So far, the studies have been conducted that heat resistance and adhesion may be imparted to a battery separator by providing a porous layer with a polyolefin microporous membrane, for example. Polyamideimide resins, polyimide resins, and polyamide resins, which have good heat resistance, fluorine-based resins, which exhibit good adhesion, and the like are preferably used as resins constituting porous layers. Alternatively, a water-soluble or water-dispersible binder has been used in recent years, which can be used to form a multi-layer structure of porous layers using a relatively easy step.
Note that, in the present specification, porous layers are the layers obtained by a wet coating method.
In Example 5 in Patent Document 1, a multilayer porous membrane of a total thickness of 24 μm (coating thickness 4 μm) is obtained by coating an aqueous solution of uniformly dispersed titania particles and poly vinyl alcohol on a polyethylene microporous membrane of 20 μm thickness by a gravure coater, where the polyethylene microporous membrane is produced by the simultaneous biaxial stretching method, and drying the coated polyethylene microporous membrane at 60° C. to remove water.
In Example 3 in Patent Document 2, a multilayer porous membrane of a total thickness of 19 μm (coating thickness 3 μm) is obtained by coating an aqueous solution of uniformly dispersed titania particles and poly vinyl alcohol on a polyethylene microporous membrane of 16 μm thickness by a bar coater, where the polyethylene microporous membrane is produced by the simultaneous biaxial stretching method, and drying the coated polyethylene microporous membrane at 60° C. to remove water.
In Example 1 in Patent Document 3, the multilayer porous membrane is obtained by the same method described in Example 3 in Patent Document 2, except that a gravure coater is used.
In Example 6 in Patent Document 4, a polyethylene microporous membrane obtained by a sequential biaxial stretching method is passed through between Meyer bars, on which an appropriate amount of a coating solution containing meta-type wholly aromatic polyamide, an alumina particle, dimethylacetamide (DMAc) and tripropylene glycol (TPG) is provided, then subjected to coagulation, water washing, and drying steps, to obtain a non-water-based separator for a rechargeable battery, in which a heat-resistant porous layer is formed.
In Patent Document 5, a polyethylene microporous membrane obtained by a sequential biaxial stretching method is passed through between facing Meyer bars, on which an appropriate amount of a coating solution containing meta-type wholly aromatic polyamide, aluminum hydroxide, dimethylacetamide and tripropylene glycol is provided, then subjected to coagulation, water washing, and drying steps, to obtain a non-water-based separator for a rechargeable battery, in which a heat-resistant porous layer is formed.
In Patent Document 6, a polyethylene microporous membrane obtained by a sequential biaxial stretching method is passed through between facing Meyer bars, on which an appropriate amount of a coating solution containing polymetaphenylene isophthalamide and an aluminum particle, dimethylacetamide (DMAc) and tripropylene glycol (TPG) is provided, then subjected to coagulation, water washing, and drying steps, to obtain a non-water-based separator for a rechargeable battery, in which a heat-resistant porous layer is formed.
In Patent Document 7, a non-porous membrane-like material of three-layer structure, having a layer including polypropylene containing a β crystal nucleating agent as an outer layer, is longitudinally stretched using a longitudinal stretching device, then coated with an aqueous dispersion including an alumina particle and poly vinyl alcohol using a Meyer bar, stretched in a transverse direction at a magnification of 2, and subjected to heatsetting and relaxation treatment to obtain a multilayer porous film. The process is, namely, a combination of the sequential biaxial stretching method and in-line coating method.
In Patent Document 8, an example of a separation membrane is described. The separation membrane is obtained by a sequential biaxial stretching method, in which an angle of contact between the material to be stretched and a stretching roller is defined to be equal to or larger than a certain value, in a longitudinal stretching device configured with four stretching rollers.